1. Field of the Invention
This invention relates to acoustic projectors and more particularly to a projector having an openwork honeycomb-like shell.
2. Brief Description of Prior Developments
Low frequency flexural sonar projectors have been developed for a variety of subsea and seismic prospecting applications. Several types of projectors which have been developed over time include flextensional projectors, inverse flextensional projectors, slotted cylinder projectors, oval-shaped projectors and others. The type of projector selected for a specific application has been dependent on the specific application requirements. Typical requirement parameters are source level, frequency, size, weight, bandwidth, efficiency and operating depth. Generally, projector development has evolved to produce low frequency sonar at greater source levels, bandwidths and efficiencies in a smaller lighterweight package for greater ranges of depths.
Current emphasis is on weight and cost reduction while maintaining or enhancing acoustic performance. There is thus a need for lower cost sonar projectors that weigh less than current projectors, with reduced weight allowing for a sonar system to be installed on smaller more agile ships that provide greater options when deploying the acoustic projectors.
Importantly, in the field of low frequency sonar projectors, nearly all projector shells used to date have been solid, and have been made of materials such as aluminum, steel, fiberglass and graphite composites. Recent sonar development has focused on the use of graphite composites because of desirable acoustic properties attributed to its relatively high specific stiffness. However, grades of this composite material best suited for sonar applications are expensive. There is thus a requirement for a structure in which the volume of material is reduced which results in lower material cost.
Moreover, there is a need for lighter projectors in general for ease of storage and development so that projector arrays can be carried aboard smaller and smaller vessels. Assuming satisfactory acoustic properties, lighter shells can give rise to either higher power density projectors or conversely a lighter weight projector for the same power output. Additionally, there is a need for greater bandwidth so as to increase resolution to permit finer target identification or application to a wide range of sonar return measuring scenarios. There is also a need for lower cost materials, such as aluminum, that can be configured to provide the equivalent stiffness of a solid graphite composite structure.
More generally, as a result of changing world political and military alignments, mobile surveillance naval operating forces face an increased requirement to carry out low frequency active, LFA, missions. Lightweight miniaturized acoustic sources that enhance mission effectiveness by increasing area searched at reduced cost are thus extremely desirable. These sources also need to have wide frequency bandwidth, high reliability, and high acoustic power densities, and must be affordable to meet their area search requirements.
The potential for dramatically increasing the acoustic power density of low frequency active sonar has already been demonstrated by slotted cylinder projectors with power densities of approximately 300 watts/Kg-KHz-Qm. Further, acoustic power densities of up to 800 watts/Kg-KHz-Qm are possible with slotted cylinder projectors configured with high strain lead magnesium niobate ceramic material. These greater projector power densities allow for greater sonar system and mission flexibility such as greater detection ranges, bandwidth, source miniaturization, weight and size reductions.
Recently, slotted cylinder projectors have operated at various electric fields and stresses in excess of 100 million cycles, a significant achievement over the previous state-of-the-art devices. Generally the slotted cylinder projectors have mechanical quality factors, Qm's of 5–6, where Qm=freq of resonance divided by the difference in frequency of the one-half power frequencies. Lower Qm's are desirable to accommodate a wide range of frequencies, and have been achieved by clustering projectors in close proximity. However, for towed projector arrays used for either surveillance or tactical missions, this can result in increasing the frontal drag area of the projector cluster which can limit tow speeds due to increased drag loads, or result in reduced mission duration due to higher fuel consumption. If one could enhance the bandwidth of the single element projector, one could reduce the need for clustering and the attendant hydrodynamic issues. With reduced drag, operating speeds can be increased, increasing the area searched and improving mission effectiveness. While reducing the number of projectors minimizes array drag, there is also a need to reduce the weight of the projectors, as well as minimizing the need for heavy array stabilizers so that the projectors can be stored on even smaller craft.